AAc blocks instead of Vermiculite insulation under slab

[Ed_]

Re: AAc blocks instead of Vermiculite insulation under slab

I am also a geek by trade, and have been speaking to geek friends about this. I had thought to embed rather more thermocouples than necessary (probably something like Carl did) and I'm toying with the notion of automated data logging. As many have pointed out, this is entirely unnecessary for successful baking, but when you're a geek it's hard to resist the temptation.

So I think it's neat! And if it actually helps quantify the benefits of this or that insulation, that's good too.

Ed

[Wlodek]

Re: AAc blocks instead of Vermiculite insulation under slab

Carl,

Thanks for the kind words, that's what I've been trained to do (system identification and control).

I did mention that this is a very approximate result - I won't bother all with detailed statistics unless there is need. The accuracy will improve if we have better data.

The model is a simple one really: heat goes from where it is hotter to where it is cooler at the rate proportional to the temperature difference and dependent on the material parameters and what is called boundary conditions (insulation, weather conditions, heat source etc.)

This simpler model is quite easily estimated from data like yours. Its simplicity makes it more reliable, but sufficiently precise for our purposes of estimating heat loss through the insulation of the dome (and by extrapolation - through the floor).

The simple model is also in contrast with large models made up of sets of equations including combustion processes, turbulent convection, interaction with the atmosphere etc, which also require very detailed and expensive measurements if they are to be of any practical use. They would work too, and potentially "better", but would also produce more information than we need and cost rather a lot more.

Limitations: this model does not include your heat input, but the easily measured heat transfer from the inner to the outer surface of the dome, so what we can estimate is the losses through the external insulation. So for example losses through the door are not included, as they influence the inner surface of the dome.

If you could collect the same measurements, soon (while the oven is still a little damp) and later, we can measure how the drying process influences the losses, if you want to go that geeky.

How to measure? You did the first record just as it should be done, but to find the losses with less uncertainty I would need the samples on a longer stretch of approximate "steady state". In the current data set the outer surface temperature does not quite reach this level.

I'll be playing with things like this when my oven is finally finished, but it would be interesting to compare how the differences in materials and techniques influence heat loss. Are the differences significant? Are dearer materials justifiable? Is Forno Economico economico? I suspect it is.

We could make a quantified contribution possibly of interest to many on the the F-B Forum to whom I am feeling greatly indebted for all I learnt here.

What do you think? What do others think?

W.

[Carl]

Re: AAc blocks instead of Vermiculite insulation under slab

W

That's clever stuff. You're understanding this on a whole different level to me. I'm surprised it's only 5-7% - I had imagined much more. This is only loose fill vermiculite, and no door.

Tell you what - let me collect a bit more data and I'll send it all through to you by PM. You're entirely right about the steady state temperature difference being the key thing, I wasn't at my most eloquent when I posted the first post...

I need to start a fire first thing in the morning and look at the temperature changes over the day - I'm also keen to see how much heat is retained once I get a door on the thing. I'll get back to you - but seriously - wow - the expertise on this forum!

Thanks, Carl

[Wlodek]

Re: AAc blocks instead of Vermiculite insulation under slab

Carl,
I digitised your posted plot ...
And this is a very rough result because the data series only cover a growing temperature period of the outside dome surface (including the tipping point of the outside curve would make it much more precise), but I can say that using a first order dynamic approximation of the heat transfer equation fitted to your data I obtained the rough estimate of your temperature loss at approximately 5-7% . Does this make sense? How does this compare with general assessment of such losses?

W.

[david s]

Re: AAc blocks instead of Vermiculite insulation under slab

I used some of this insulating concrete for my mobile oven (see pic in finished ovens) The product is called Hebel in Australia, owned by CSR. The stuff I used is 75mm (3") thick with 1/4" steel reinforcing cast into it. Strong and light. Because it is made of Portland cement it's probably no good as a hot face.

[Wlodek]

Re: AAc blocks instead of Vermiculite insulation under slab

The faster the outside heats up (relative to the inside) the better my insulation (vermiculite) is working.

From what I remember what tells you more about the quality of insulation is how close the two get in the steady state (when it has all equilibrated), not how fast they do it.

If they are very diferent in steady state it means that a lot of heat is lost. If they get identical, then the insulation is perfect. In other words, the closer this steady state coefficient is to 1, the better. If it is say 0.99, then it (roughly) means that 99% of the heat stays in ...

It awkward and expensive to run an experiment for a very long time with steady heat input, but I should be able to estimate this relation (steady state) if you send me (or post) the data you plotted. The longer the series and the more varied the signals are - the better. Changing the level of the heat input, or even recording the entire firing session would be ideal, even if the hot steady state is not reached.

I am sure someone will correct me if I am wrong. And of course we can organise a Steady State Gain Challenge ...

Incidentally, I should be starting digging for the foundations of mine in two weeks, weather permitting. It all got delayed because I can not start until all other work (kitchen, patio roof and patio paving) is finished, and it all got delayed by a few months. We are doing the last bits of the patio next weekend. All the materials for the oven are sat in the garage. Very exciting!

Cheers,

W.

[Carl]

Re: AAc blocks instead of Vermiculite insulation under slab

W.

You're right - it's exactly as you describe it. Obviously the inside of the dome gets hot first, and the heat is gradually distributed across the 4" of clay. I put the thermocouple there so I can see when the entire thickness of the dome is saturated. The faster the outside heats up (relative to the inside) the better my insulation (vermiculite) is working. Hopefully better, when it's all dried out...

[Wlodek]

Re: AAc blocks instead of Vermiculite insulation under slab

Carl,
I understand that the "dome outside surface" sensor is under all the vermiculite etc. and measurements there will reflect heat saturation, not heat loss. Please correct me if I am wrong.
W.

[PeterW]

Re: AAc blocks instead of Vermiculite insulation under slab

The blocks on the base are now rendered - that was yesterdays job before we had the torrential rain today !

The top has very little left showing - it may get rendered, or it may have some sort of tile applied after the brick skin goes on. You can see from the photo that theres not a lot showing.

The plan is to build the brick outer dome as the waterproof layer, and then probably coat it in something like Thompsons Water Seal or the like. I've done that before, and its good for keeping it dry

Cheers

Peter

[Carl]

Re: AAc blocks instead of Vermiculite insulation under slab

I'm sure that plenty of other people have had the idea too - so you're not nicking an idea from me! I got the idea from other posts that were already here when I arrived.

Make sure that your thermalite blocks are well protected from the weather - they soak up water like sponges, and they won't do any insulating if they're wet. I rendered mine, then painted the render. I'd imagine your planned brick skin will help. I presume you're going for some sort of roof over your oven?

[PeterW]

Re: AAc blocks instead of Vermiculite insulation under slab

I've shamelessly nicked Carls idea, and used Thermalite blocks for an insulated hearth.

The photos attached show my basework, ready for the oven chamber build to start next week. I've used Thermalite all round for the structure, added a 4" reinforced concrete slab and poured 3" of insulcrete. I wasn't happy with the vermiculite/cement mix, so I've added this top layer of blocks. They are laid staggered, and on a dry sand/cement mix. I've brushed a dry mix in the small gaps between the blocks and watered the blocks to start the setting process.

The thermocouples will be drilled into the base so I'll wait and see what temperatures we end up with !

Cheers

Peter

[Carl]

Re: AAc blocks instead of Vermiculite insulation under slab

I'm hoping the outside of the dome will hold more heat once everything's dried out a bit - it's got a minimum of 6" loose vermiculite plus 4" of AAC block around the dome at the bottom - and more like 2 foot of loose vermiculite on the top... I'll post another graph once I've got a door and it's dried out a bit.

[jengineer]

Re: AAc blocks instead of Vermiculite insulation under slab

That is very good indeed Very little loss from the bottom - any chance of getting more insulation on the outside of the dome? oh yeah, its a done deal...never mind

[Carl]

Re: AAc blocks instead of Vermiculite insulation under slab

I promised to post my results with the AAC block - so here goes. The graph below is after a gentle 3 hour fire, and the 'below hearth insulation' line is the thermocouple that's below the AAC block that's below the hearth bricks. I'm pretty pleased with that - only getting to ~25 degrees above ambient.

I'll post more from larger fires when I have them. I also need to make a door to fit the oven - that's why it's cooling so quickly!

Carl

[vtbread]

Re: AAc blocks instead of Vermiculite insulation under slab

No problem, glad the insulating properties were cleard up.

I have a pallet full of the stuff left over from a house building project most are 8"x12"x 24" should make a pretty nice insulation layer.

I just poured my slab for the oven and shed and am working on the superstucture now. Get a roof on it first, tent in and then I can focus on the oven. It's a bit nippy here right now.

I'll post some pics as I progress.